1. Field of the Invention
This invention relates, generally, to cryogenic power cables or superconducting power cables. More particularly, it relates to terminations used in medium voltage (<˜63 kV) power cables (e.g., superconducting cables) and methods of maintaining temperatures and integrities thereof.
2. Description of the Prior Art
A high voltage cable termination has the purpose to allow an electric connection of a cable with connectors, conductors, or bus bars. The termination must allow dielectric integrity and thermal management of a cable at any point of operation, in particular for any permissible load current and ambient temperature, when cooled by a gaseous cryogen. The termination must also maintain the conductor below a critical temperature if superconducting materials are used.
Dielectric integrity is challenging in systems with gaseous cryogen used as both coolant and electric insulation. Gaseous media have a low dielectric strength compared to liquid media. However, gaseous media are considerably better at cryogenic temperature and elevated pressure compared to standard temperature and pressure (STP) conditions. It is of paramount importance for the dielectric integrity of the cable termination that any gaseous cryogen is at low temperature and high pressure, particularly on the solid insulator surfaces and at the location of the superconductor.
Careful thermal management is critical in order to keep the conductor in the cable cold (i.e., minimize heat influx to the cable) and at the same time minimize the required power to cool the termination and the cable. Thermal management is interconnected with dielectric integrity since the temperature of the gaseous cryogen is critical for dielectric integrity.
Power cables made of high temperature superconductors (HTS) have been of interest because of the availability of the materials in long length tapes [1, 2, 3]. Many prototype demonstrations of HTS power cables have been successfully completed. A majority of the demonstrations were AC power cables cooled with liquid nitrogen.
However, long distance superconducting direct current (DC) power transmission cables were found to be advantageous [4, 5, 6, 7]. One of the attractions of DC transmission systems is their suitability for smart grid concepts where power can easily be transmitted from large scale renewable energy sources, such as wind farms, photovoltaic farms, and ocean wave power generators, as well as interconnection between independently-controlled power grids and long distance power transmission. Another advantage of superconducting DC transmission and distribution systems is the absence of AC losses in the cables that add to the cryogenic capacity required to operate the cables. There have been a limited number of demonstration studies of superconducting DC power cables cooled with liquid nitrogen.
More recently, there has been research exploring the possibility of using cryogenic helium gas circulation for cooling HTS power cables [8, 9]. The primary advantage of helium, as compared to liquid nitrogen, is the possibility of lower operating temperatures that allow the cable designs to take advantage of the significantly higher critical currents of HTS tapes, thus allowing the cables to be smaller in size and weight. Reduced size and weight are particularly attractive for naval and aviation applications [10, 11, 12, 13].
The United States Navy, National Aeronautics Space Administration and Air Force Research Laboratory have been investigating all-electric ships and all-electric airplanes based on superconducting cables, motors, and generators [13]. Lower operating temperatures are also essential for cables based on MgB2 superconductors whose superconducting transition temperature is around 39 K and typical cable operating temperatures are between 10 K and 20 K [9]. Circulation of gaseous or liquid neon and hydrogen are potential alternatives, but for cost and safety reasons, gaseous helium circulation has been the choice for ongoing research of superconducting cables [10, 11, 12].
Superconducting DC cables typically do not generate any heat, though different cryogens have different heat capacities and thermal conductivities, thus having a substantial impact on the thermal aspects of the cable. Superconducting degaussing cables cooled with gaseous helium circulation have successfully been demonstrated [10, 11]. A power transmission or distribution cable system consists of the terminations that transfer several kiloamperes of current from the network at ambient temperature to the superconducting cable at cryogenic temperatures. The heat from the ambient through the current leads and the termination tanks, along with the Joule heating (the generation of heat by passing electric current through a conductor) at the resistive joints between the external current leads and the internal terminals (e.g., aluminum, copper, other metal alloys contributing to Joule heating) of the superconducting cable, is significant.
This heat load at the terminations is one of the most challenging design aspects of helium gas cooled superconducting cables. Another design challenge for helium gas cooled superconducting cables is the substantially lower dielectric strength of helium gas [14, 15, 16]. The challenge is exasperated at the terminations because the warmer interfaces reduce the local gas density, thus making the dielectric strength weaker [16].
Accordingly, what is needed is an apparatus and method for more efficiently maintaining thermal and dielectric integrity in cable terminations, in particular those of gas cooled superconducting cables. However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill how the art could be advanced.
All referenced publications are incorporated herein by reference in their entirety. Furthermore, where a definition or use of a term in a reference, which is incorporated by reference herein, is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
While certain aspects of conventional technologies have been discussed to facilitate disclosure of the invention, Applicants in no way disclaim these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein.
The present invention may address one or more of the problems and deficiencies of the prior art discussed above. However, it is contemplated that the invention may prove useful in addressing other problems and deficiencies in a number of technical areas. Therefore, the claimed invention should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein.
In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.